1. Field of the Invention
The present invention relates to microwave amplification tubes, such as traveling wave tubes or klystrons, and more particularly, to an integral polepiece RF amplification tube having enhanced gain and transmission.
2. Description of Related Art
Microwave amplification tubes, such as traveling wave tubes (TWTs) or klystrons, are well known in the art. These microwave tubes, are provided to increase the gain, or amplify, an RF (radio frequency) signal in the microwave frequency range. A coupled cavity TWT typically has a series of tuned cavities which are linked or coupled by irises formed between the cavities. A microwave RF signal induced into the tube propagates through the tube, passing through each of the coupled cavities. A typical coupled cavity TWT may have up to thirty individual cavities which are coupled in this manner. The meandering path which the RF signal takes as it passes through the tube reduces the effective speed of the traveling signal so that it can be operated upon. The reduced velocity wave formed by a coupled cavity tube of this type is known as a "slow wave."
Each of the cavities is further linked by a beam tunnel which extends the length of the tube. To produce an amplified RF output signal, an electron beam must be projected through the beam tunnel. The beam is guided by magnetic fields which are formed in the tunnel region. The electron beam will interact with the RF signal to produce the desired amplification. The bandwidth of frequencies of the resulting RF output signal can be changed by altering the dimensions of the cavities, and the strength of the RF output signal can be changed by altering the voltage and current of the beam.
The magnetic field which is induced in the tunnel region is obtained from flux lines which flow radially through polepieces from magnets lying outside the tube region. The polepiece is typically made of magnetic material, which channels the magnetic flux to the beam tunnel. This type of electron beam focusing is known as Periodic Permanent Magnet (PPM) focusing. An RF amplification tube can either utilize an "integral polepiece" or a "slip-on polepiece." An integral polepiece forms part of the vacuum envelope extending inward towards the beam region, while a slip-on polepiece lies completely outside the vacuum envelope of the tube. When the polepieces form part of the tunnel as well as the cavity wall, the magnetic flux in the beam region can result in large beam stiffness values, or .lambda..sub.p /L (where ".lambda..sub.p " is the wavelength of the plasma frequency of the beam and "L" is the period of the sinusoidal function of the magnetic field in which the beam propagates), a desirable condition for focusing beams. For this reason, integral polepiece RF amplification tubes are preferred over slip-on polepiece tubes.
Klystrons are similar to coupled cavity TWTs in that they can comprise a number of cavities through which an electron beam is projected. The klystron amplifies the modulation on the electron beam to produce a highly bunched beam containing an RF current. A klystron differs from a coupled cavity TWT in that the cavities are not generally coupled. A portion of the klystron cavities may be coupled, however, so that more than one cavity can interact with the electron beam. This particular type of klystron is known as an "extended interaction output circuit."
A significant problem with RF amplification tubes is the efficient removal of heat. As the electron beam drifts through the tube cavities, heat energy resulting from stray electrons intercepting the tunnel walls must be removed from the tube to prevent reluctance changes in the magnetic material, thermal deformation of the cavity surfaces, or melting of the tunnel wall. To remove the heat, copper plates are usually joined to the portion of the magnetic material that conducts the heat to the heat sink. The use of copper lowers the thermal resistance of the heat path and more easily keeps the tunnel temperature below dangerous levels. The minimum thermal path length in typical cylindrical cavities is the radius of the cavity.
An additional problem with RF amplification tubes is that it becomes more difficult to construct them to amplify RF signals in the millimeter wavelength range of the microwave spectrum, or millimeter waves. These extremely short wavelength signals require precise tolerances in the formation of the cavities and the coupling irises. It is well known that in a periodic microwave structure, an increase in the period-by-period variation of the inside dimensions (seen by the RF fields), will result in an increase of RF reflections inside the tube. This, in turn, results in degraded impedance matches between the tube and the RF input waveguide, and lower periodicity values than would otherwise exist. These factors result in reduced gain values achievable by the tube. Thus, as the nominal dimensions of parts decrease with the higher frequencies, the size of the period-by-period variations must also decrease.
In prior art integral polepiece RF amplification tubes, magnetic and non-magnetic parts are usually machined individually, stacked, then brazed together. In tubes designed to operate at millimeter wavelengths, the period-by-period dimensional variations are often determined not only by the tolerances called out for the individual parts, but also by non-uniformities of the braze regions between the parts. At higher frequencies, where more periods and hence more parts are usually required, it becomes more difficult or costly to avoid tolerance build-up along the stack, especially if copper plates must be added to the polepieces to improve the thermal conductivity along the cavity wall.
Consequently, integral polepiece RF amplification tubes become less useful as the operating frequencies and the number of parts increase. More often, the tube is machined out of a single block of copper using discharge machining technique to control the dimension variation problem. Afterwards, a separate magnetic circuit is slipped on and brazed to the tube if light weight PPM focusing is desired. However, by eliminating the integral polepiece, and the consequent introduction of magnetic flux at the tunnel wall, the desirable focusing property of integral polepiece RF amplification tubes has been lost. The ratio of .lambda..sub.p /L is significantly reduced, and only higher beam voltages can be focused.
Another consideration with PPM focusing systems is the relationship between beam tunnel diameter and separation between centers of adjacent polepieces. Generally, a relatively small diameter beam tunnel is desired since it presents better interaction impedance with the electron beam, resulting in greater RF output power and gain. In integral polepiece PPM focusing systems, the iron of the polepiece can extend towards the beam axis so as to form part of the beam tunnel or be very close to the beam tunnel. In such cases, the polepiece geometry typically maintains a ratio of: EQU d/P&lt;1
in which d is the diameter of the hole in the iron polepiece (or the beam tunnel diameter) and P is the separation between centers of adjacent polepieces. Slip-on polepiece PPM focusing systems often have a ratio of hole diameter to polepiece separation of greater than one, however, the interior region of the beam tunnel used by the beam is usually near the axis of the system.
In focusing an electron beam, the magnetic field strength at the edge of the beam is of primary significance. Electron beams are often defined in terms of the ratio of the effective radius of the beam and the beam tunnel radius, known as the electron beam "fill factor." An electron beam fill factor of 0.6 is considered typical. PPM focusing systems utilizing the geometric relationship defined above tend to exhibit very small RMS axial magnetic field variation across the beam tunnel diameter. While this is acceptable for ideal electron beams having relatively smooth electron motion with no radial velocity component, imperfect electron beams are not so efficiently focused. An imperfect beam may exhibit electron excursions that impinge on the beam tunnel wall, generating excess heat and reducing the efficiency of the RF amplification tube.
Beam tunnel size also has an effect on the gain achieved by the RF amplification tube. Gain of a propagating RF wave in a traveling wave tube is proportional to the normalized transverse wave number, .gamma.a, where .gamma. is the radial phase constant of the wave, and a is the radius of the circuit on which the RF wave propagates, in this case, a is the radius of the beam tunnel. In PPM focusing systems at high frequencies, a small beam tunnel radius is considered essential for effective interaction between the electron beam and the propagating RF wave, and gain generally decreases when .gamma.a becomes too large. The normalized transverse wave number is also proportional to 2.pi./.lambda., in which .lambda. is the wavelength of the propagating RF wave, and is a measure of the size of the RF wave with respect to the beam tunnel. For large values of .gamma.a, the RF electric and magnetic fields fall off rapidly away from the beam tunnel surface. Thus, in actual practice, PPM focusing systems generally select .gamma.a to be less than 2.2 in order to achieve a useful gain level.
Thus, it would be desirable to provide an integral polepiece RF amplification tube for amplifying a millimeter wave RF signal having polepieces extending fully, or at least partially, to the tunnel wall to provide desirable beam focusing. It would also be desirable to provide an integral polepiece RF amplification tube having copper plates in contact with the polepieces along the cavity wall to improve heat removal from the tunnel wall. It would be further desirable to provide a relatively inexpensive method of fabricating an integral polepiece RF amplification tube having the aforementioned features and which eliminates the deleterious effects of tolerance build-up. It would also be desirable to provide an integral polepiece PPM focusing system that has greater RMS magnetic field strength at the outer portion of the beam tunnel for more efficient focusing of the electron beam.